It is now well known that ultrasonic echoscopy techniques can be used to provide information about an object that is not visible to the eye. The basic technique of ultrasonic echoscopy involves directing a short pulse of ultrasonic energy, typically in the frequency range from 1 MHz to 30 MHz, into the region of the object that is being examined, and observing the energy that is reflected, as an echo, from each acoustic impedance discontinuity in that region. Each echo received is converted into an electrical signal and displayed as either a blip or an intensified spot on a single trace of a cathode ray tube or television screen. Such a display of the echoes is known as an "A-mode" echograph or echogram, and is useful in a number of diagnostic techniques to locate the boundaries of the object or to provide other information about the region into which the pulse of ultrasonic energy has been directed.
If a series of adjacent A-mode displays are obtained (for example, by physically or electrically moving the transmitting transducer which produces the pulses of ultrasonic energy, or by scanning the direction of transmission of the pulses of ultrasonic energy), a two-dimensional image of the object under examination may be displayed on the cathode ray tube or television screen. Such an image or display of acoustic discontinuities is known as a "B-mode" image or display.
The use of the Doppler frequency shift in the ultrasonic examination of flowing liquids and moving objects is also well known. Many echoscopes which perform the B-mode imaging examination described above can also perform Doppler frequency shift measurements in respect of echoes returned from moving objects within the region receiving ultrasonic energy from the echoscope When the object under examination is a blood vessel, measurement of the Doppler shift of echoes from the blood cells within the vessel permits the velocity of those blood cells to be estimated. As pointed out by R W Gill, in his article entitled "Measurement of Blood Flow by Ultrasound: Accuracy and Sources of Error", which was published in Ultrasound in Medicine and Biology, Volume 11 (1985), pages 625 to 641, it is possible to measure the total volume of flow per unit time using an ultrasonic examination technique which includes the measurement of frequency changes due to the Doppler effect.
In ultrasonic examinations including Doppler frequency shift measurements, it is necessary to obtain echoes from a limited volume of the flowing liquid which is within the vessel being examined. This is achieved by fixing the line of sight of the ultrasonic transducer and, in the most commonly used version of Doppler measurement known as "pulsed Doppler", analysing the echoes obtained from the sample volume for a limited range of time delays. The Doppler shift in the received echoes is averaged in order to calculate the average speed of scatterers in the flowing liquid.
In current applications of the pulsed Doppler technique, a small sample volume within the vessel is selected by the operator of the echoscope, who moves a graphical representation of the sample volume over a B-mode image of the vessel. In this way, the B-mode imaging equipment is used to steer the ultrasonic beam and adjust the sample volume delay so that the actual sample volume position corresponds to that part of the vessel which is to be the subject of the Doppler shift measurement. The orientation of the vessel has to be known, so that the velocity of the liquid within the vessel may be calculated from the well-known Doppler equation: ##EQU1## where f.sub.D is the Doppler shift frequency, f.sub.o is the transmitted frequency, v is the blood velocity, c is the speed of sound and .theta. is the angle between the line of sight of the ultrasonic beam and the direction of flow of the liquid. In current implementations of this technique, the orientation of the vessel is obtained from observations of the graphical representation of the sample volume in the B-mode ultrasonic image.
In the volumetric measurement of flow, a larger sample volume is placed to encompass the entire vessel, and the total flow is calculated using the relationship: ##EQU2## where d is the vessel diameter and f.sub.D is the mean Doppler shift in frequency. When applying this formula, the diameter of the vessel is estimated by the operator, who identifies the positions of the two internal vessel walls on the B-mode image and places cursors on their images. The diameter of the vessel is taken as the distance between the cursor positions. This is a difficult measurement, and because the flow is directly proportional to the square of the diameter in the expression for flow, errors in the diameter measurement translate into greater errors when the flow values are estimated.
Another factor affecting the accuracy of blood flow measurements is the fact that, in humans and animals, the diameter of most vessels varies during the cardiac cycle. This is particularly so in the case of arteries. Hence, for greatest accuracy, the instantaneous values of f.sub.D and d should be obtained repeatedly and the expression in equation (2) should be averaged over several cardiac cycles.